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Heart As A Pump And Cardiac Cycle Mechanical events : The duration of one cardiac cycle is approximately 0.8 second, making a heart rate of 75 beats/minute, (we use these values for studying purposes, but they actually vary according to the heart rate which ranges between 60 – 100 beats/minute in normal cases). The mechanical events that happen during one cardiac cycle beginning from atrial systole: 1) Atrial systole takes 0.1 second, and the atrial diastole takes the rest of the cycle‟s time, 0.7 second. 2) The AV delay makes the ventricles contract after the atria finish their contraction, so the ventricular systole begins at 0.1 second on the diagram, its duration is 0.3 second and therefore it finishes at 0.4 second. Ventricular diastole obviously takes 0.5 second, and it lasts from 0.4 to 0.9 second on the diagram. AS AS VS 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 *continuous line means systole, dotted line means diastole, the one above is for the Atria and the one below is for the ventricles. - The atria contract while the ventricles are in diastole, as seen in (00.1) and (0.8-0.9). 1|Page The electrical response precedes the mechanical one, so the P wave is recorded before the atrial systole, and QRS before the ventricular systole, also T precedes ventricular diastole in a way similar to the diagram below. P R Q 0 S 0.1 T 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 We can add the action potential on the diagram, (you‟re supposed to understand the relationship between the action potential and ECG from previous lectures). atrial 2|Page ventricular action potential Volume changes : Now we‟ll talk about volume changes, we don‟t care about the atrial volume, the ventricular volume is the important one. The blood volume of the right ventricle equals that of the left ventricle, the ventricles differ only in pressure (pressure in the left > right). Before the atria contract, there is a certain amount of blood in the ventricles, let‟s say there‟s 100 ml in the left ventricle before the atrial contraction, now when the atria contract, the AV valves would be already open (remember that they open passively due to pressure gradient at the beginning of ventricular diastole). The atrial pressure is almost 0, the ventricular pressure is almost -2 mmHg (negative, because we‟re measuring relative to the atmospheric pressure which is our reference, so a value of zero here equals the atmospheric pressure = 760 mmHg). When the atria contract, they push certain amount of blood to the ventricles, so the volume in the left ventricle (our example) becomes 125 ml at the end of the ventricular diastole, we call it End Diastolic Volume “EDV”. The atrial systole contributed to about 25% of this volume. if they didn‟t contract (as in atrial fibrillation), blood would also be flowing to the ventricles because the AV valves are open, but they will move with less amounts and speed in this case (the volume in the ventricle would increase by 15% or 10% only.) At the beginning of ventricular systole, pressure will gradually build up in the ventricles, exceeding that of the atria, when blood tries to move from the ventricles to the atria, the AV valves will close. At this stage, the 4 valves of the heart (AV & semilunar) are closed, and the volume in the ventricle doesn‟t change, therefore we call this phase Isovolumic contraction, which is a short phase in which ventricular volume is 3|Page constant and the 4 valves are closed, and with contraction, the pressure sharply increases. When the pressure in the left ventricle exceeds that of the aorta, the aortic valve opens, and the blood is ejected from the ventricle, the first ejection phase is called the rapid ejection, after it the blood is ejected slowly. At the end of systole, 55 ml stays in the ventricle and it‟s called the End Systolic Volume “ESV”, the amount of blood ejected in each beat (cardiac cycle) from either right or left ventricle is called stroke volume, and here it equals (125 – 55 = 70 ml/beat). Stroke volume = EDV – ESV. During ventricular diastole, pressure in the left ventricle starts to decrease, when it becomes less than the aortic pressure, blood tries to move from the aorta to the left ventricle, closing the semilunar / aortic valve, and the AV valves are still closed, and this diastolic phase where the 4 valves are closed, is called Isovolumic relaxation. The pressure in the Isovolumic relaxation phase keeps decreasing until it becomes lower than the atrial pressure, then the AV valve opens rapid filling then slow filling. Before the atrial systole, the blood volume in the ventricle rises to 100 ml (passively by pressure gradient), when the atria contract, another 25 ml are added. * Cardiac output (ml/minute) = stroke volume X heart rate The stroke volume in the left ventricle equals that in the right ventricle, they have to be so ! Let‟s suppose that the stroke volume in the right ventricle is 70 ml/beat, and in the left ventricle 69ml/beat, consequently, 1 ml accumulates in the left ventricle per beat, if the heart rate was 70 beat/minute, 70 ml will accumulate every minute, 4|Page 4200 ml every hour! this is called „heart failure‟. The left ventricle normally pumps what it receives from the right ventricle, even if a small amount is accumulated in one beat, compensation will happen in the next one. Pressure changes : Left ventricular pressure during ventricular diastole equals -2 or -1 or 0 (doesn‟t matter), while the aortic pressure is 80 mmHg. Because of the atrial systole, the left ventricular pressure increases a little bit (rises to be around +3 mmHg), when the isovolumic contraction starts, left ventricular pressure increases sharply, until it exceeds 80, causing the aortic valve to open, and it keeps rising to be higher than the aortic pressure. 5|Page At the last part of the ventricular systole, the aortic pressure will be higher than the left ventricular pressure, although the blood is still moving from the ventricle to the aorta, and that happens because the blood has a momentum, when the force of the aortic pressure overwhelms the force of the blood‟s momentum, it closes the aortic semilunar valve, starting the isovolumic relaxation, the left ventricular pressure decreases until we reach the end of this phase where ventricular pressure falls to be lower than atrial pressure and the AV valves open. When the aorta pushes the blood closing the semilunar valve, the blood is compressed; so aortic pressure slightly rises, making a small peak, called dicrotic notch or incisura. The pressure in the left ventricle varies between 0 – 120 mmHg, in the aorta it varies between 80 – 120 mmHg. The right ventricular pressure varies between 0 – 25 mmHg, in the pulmonary artery 8 – 25 mmHg. If we want to make a similar diagram for the right side, the waves (below) are the same, but we just change the scale (pressure values). Heart sounds : The 1st heart sound “lup” (S1) appears when the AV valves close. The 2nd heart sound “dup” (S2) appears when the semilunar valves close. The time between S1 and S2 represents ventricular systole (0.3 second), the time between S2 and the next S1 represents the ventricular diastole. There are S3 & S4 but we don‟t hear them normally, however; the Phonocardiograph will record them. 6|Page 7|Page